This invention relates to a two-component powder coating system by which the curing of a coating occurs at a significantly faster rate and/or at a significantly lower temperature and produces an exceptionally smooth surface. This invention also relates to thermally stable powder coating whereby white coatings do not yellow during thermal curing. This invention also relates to the electrostatic coating of metal and, in particular, to the coating of heat sensitive substrates such as wood with a coating powder and the low temperature curing of that coating to achieve a pleasing smoothness. More particularly, it relates to an improvement of the humidity resistance of such a coating.
Traditionally, coating powders have been made by the extrusion of a mixture of resins and curing agents to obtain a homogeneous mixture and then grinding the extrudate and screening the comminuted product to obtain the desired particle sizes and particle size distribution. The powder is then electrostatically sprayed onto a substrate, traditionally a metal substrate, and cured at temperatures much higher than 200.degree. F. The curing of powder coatings on heat sensitive materials such as wood, plastic, and the like has been limited by the fact that the extrusion of a mixture of a resin and a low temperature curing agent, i.e., one that is active at 200.degree. F. or less, would cause the coating powder to gel in the extruder because the extrusion typically generates enough heat to raise the temperature to 200.degree. F. or higher. It has been thought throughout the art that the curing agent must be mixed with the resin by extrusion to obtain a uniform cure and film appearance. It has also been commonly accepted that a low gloss film must be cured at a high temperature, e.g., about 300.degree. F. or higher.
In this invention, a self-curing epoxy resin is first melt- mixed (e.g., extruded) with a small amount of catalyst or with a low level of a low temperature curing agent, then chilled and ground to a obtain a first powder and classified in the usual manner. Additional amounts of the low temperature curing agent in powder form are then blended with the first powder to raise the level of curing agent while avoiding the pre-gelation problem. Surprisingly, the time required to obtain a smooth cured film is lowered significantly. It is surprising, also, that a low gloss film is achieved at cure temperatures much lower than 300.degree. F.
The blending of the first powder with the powdered curing agent to make a two component coating powder is disclosed in U.S. Pat. No. 5,714,206 which issued from an application commonly assigned herewith, and in commonly assigned, co-pending Ser. No. 09/325,873 which was filed on Jun. 4, 1999. Now, it has been found that the humidity resistance of coatings made from such two component powders is improved by inactivating polyamine domains that remain in the powder because of incomplete blending and do not come into contact with the base epoxy resin and which, otherwise, would be carried over into the cured coating.